As I write this it’s the last day of September in the U.K., which means we’re well into meteorological autumn and summer is, at least here, a distant memory. The weather is cooler and the days are getting shorter. Soon, the clocks will go back an hour, and we’ll shift from BST (British Summer Time) to GMT (Greenwich Mean Time).

Seasons in the U.K. are particularly marked because of our northerly latitude. British weather tends to be fairly mild (thanks, Gulf Stream), and it’s easy to forget just how far north we are – but a quick look at a globe makes it clear: London is actually further north than most of the major Canadian cities, while the Polar Bear Provincial Park in Ontario is roughly on the same latitude as Scotland’s capital city, Edinburgh.

Yes, I hear you say, but what on Earth (hoho) does this have to do with chemistry?

Well, a clever little piece of chemistry happens in human skin, and, if you live in the U.K., it’s about to stop. At least, until next spring.

There’s a substance in your skin called 7-dehydrocholesterol (7-DHC). It is, as the name suggests, something to do with cholesterol (which, despite its bad press, is an essential component of animal cell membranes). In fact, 7-DHC is converted to cholesterol in the body, but it’s also converted to something else.

You will have heard of vitamin D. It helps us to absorb calcium and other minerals, and if children, in particular, don’t get enough it can lead to rickets – which leads to weak bones, bowed legs and stunted growth. Vitamin D deficiency has also been linked to lots of other health problems, including increased risk of certain cancers, heart disease, arthritis and even type one diabetes.

More recently, vitamin D has been linked to COVID-19. It’s estimated that around 80-85% of people who contract COVID-19 experience mild or no symptoms, while the rest develop severe symptoms and, even if they recover, may suffer life-altering after-effects for many months. Early data suggest that patients with low vitamin D levels are much more likely to experience those severe symptoms. There’s a plausible mechanism for this: vitamin D helps to regulate the immune system and, in particular, helps to reduce the production of cytokines.

Cytokines are small proteins which are important in cell signalling, but if the body starts to produce too many in response to a virus it can cause something called a cytokine storm, which can lead to organ failure and death.

It’s proposed that having the right levels of vitamin D might help to prevent such cytokine storms, and therefore help to prevent a severe COVID-19 response. This is all early stages, because everyone is still learning about COVID-19, and it may turn out to be correlation without causation, but so far it looks promising.

One thing you many not know is that vitamin D is, technically, misnamed. Vitamins are, by definition, substances which are required in small quantities in the diet, because they can’t be synthesised in the body.

But vitamin D, which is actually a group of fat soluble molecules rather than a single substance, can be synthesised in the body, in our skin. The most important two in the group are ergocalciferol (vitamin D₂) and cholecalciferol (vitamin D₃), sometimes known collectively as calciferol.

Vitamin D₂ is found in fungi, but it’s cleared more quickly from the body than D₃, so needs to be consumed in some form daily. Mushrooms are a good source (especially if they’ve been exposed to UV light), so if you like mushrooms, that’s one way to go. Vitamin D₃ is hard to obtain from diet – the only really good source is oily fish, although other foods are fortified – but that’s okay because, most of the time, we don’t need to eat it.

Which brings us back to 7-DHC. It’s found in large quantities in the skin, although exactly how it gets there has been the subject of some debate. It used to be thought it was formed from cholesterol via an enzymatic reaction in the intestine wall and then transported to the skin via the bloodstream. But the trouble with this idea is that the blood would pass through the liver, and 7-DHC would be reconverted to cholesterol, never having a chance to build up in skin. A more robust theory is it’s actually synthesised in the skin in the first place, particularly since higher levels are found in a layer closer to the surface (the stratum spinosum) than in the deeper dermis.

Anyway, the important thing is that 7-DHC absorbs UV light, particularly wavelengths between 290 and 320 nm, that is, in the UVB range, sometimes called “intermediate” UV (in contrast with “soft” UVA, and “hard” UVC). When exposed to UVB light, one of the rings in the 7-DHC molecule breaks apart, forming something known pre-D₃, that then converts (isomerises) to vitamin D₃ in a heat-sensitive process.

In short, we make vitamin D₃ in our skin when we’re in the sunshine. Obviously we need to avoid skin damage from UV light, but the process doesn’t take long: 10-15 minutes of midday sunlight three times a week, in the U.K. in the summer, is enough to keep our levels up.

Sun exposure is by far the quickest, and certainly the cheapest, way to get your vitamin D. If you live somewhere where that’s possible.

Here’s the thing, though, if you live in the U.K., for a chunk of the year, it’s just not. I’ve pinched the graph here from my husband, whose work involves solar panels, because it makes a nice visual point.

From April – September, there’s plenty of energy available from sunlight. But look at what happens from October – March. The numbers drop drastically. And here’s the thing: it turns out that vitamin D production in human skin only occurs when UV radiation exceeds a certain level. Below this threshold? Well, no photocoversion takes place.

In short: if you live in the U.K. you can’t make vitamin D in your skin for a few months of the year. And those few months are starting… round about now.

If you want to experiment, there’s a website here, published by the Norwegian Institute for Air Research (NILU), where you can enter various parameters – month, longitude, cloudiness etc – and it will tell you how many hours during a given a day it’s possible to synthesise vitamin D in your skin.

Have a play and you’ll see that, for London, vitamin D synthesis drops off to zero somewhere around the end of November, and doesn’t restart until sometime after the 20th of January. In Edinburgh, the difference is even more marked, running from the first week or so of November to the first week of February.

It’s important to realise that it tails off, too, so during the days either side of these periods there’s only a brief period during midday when you can synthesise vitamin D. And all this assumes a cloudless sky which in this country… is unlikely.

The situation is worse still if you have darker skin because the skin pigment, melanin, absorbs UVB. On the one hand, this is a good thing, since it protects skin cells from sun-related damage. But it also reduces the ability to synthesise vitamin D. In short, wimpy autumn and winter sunshine just isn’t going to cut it.

Likewise, to state the obvious, anyone who covers their skin (with clothing or sunblock), also won’t be able to synthesise vitamin D in their skin.

Fortunately, there’s a simple answer: supplements. The evidence is fairly solid that vitamin D supplements increase blood serum levels as well as, if not better than, sunshine – which, for the reasons mentioned above, can be difficult to obtain consistently.

Now, as I’ve said many times before, I’m not a medical doctor. However, I’m on fairly safe ground here, because Public Health England do actually recommend everyone take a vitamin D supplement from October to May. That is, from now. Yes, now.

I do need to stress one point here: DO NOT OVERDO IT. There always seems to be someone whose reasoning goes along the lines of, “if one tablet is good, then ten will be even better!” and, no. No. Excessive doses of vitamin D can cause vomiting and digestive problems, and can lead to hypercalcemia which results in weakness, joint pain confusion and other unpleasant symptoms.

Public Health England recommend everyone in the U.K. take 10 micrograms per day in autumn and winter. Babies under one year should also be given 8.5–10 micrograms of vitamin D in the form of vitamin drops, unless they’re drinking more than 500 ml of infant formula a day (because that’s already fortified).

Amounts can get a little confusing, because there are different ways to measure vitamin D doses, and in particular you may see IU, or “international units“. However, if you buy a simple D3 supplement, like this one that I picked up at the supermarket, and follow the dose instructions on the label, you won’t go far wrong.

So, should you (and everyone else in your family) be taking a simple vitamin D supplement right around now? If you live in the U.K., or somewhere else very northerly, then yes. Well, unless you’re really keen to eat mushrooms pretty much every day. At worst, it won’t make much difference, and at best, well, there’s a chance it might help you to avoid a really unpleasant time with COVID-19, and that’s got to be a good thing.

But, look, it’s not toilet roll. Don’t go and bulk buy vitamin D, for goodness sake.

Until next time, take care, and stay safe.

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